U.S. patent number 6,791,868 [Application Number 10/335,671] was granted by the patent office on 2004-09-14 for ferromagnetic resonance switching for magnetic random access memory.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Savas Gider, Vladimir Nikitin.
United States Patent |
6,791,868 |
Gider , et al. |
September 14, 2004 |
Ferromagnetic resonance switching for magnetic random access
memory
Abstract
A new method of performing the write operation on the MRAM bit
cell with improved switching selectivity and lower write current
requirements is achieved utilizing oscillating word write currents
at frequency near the ferromagnetic resonance frequency of the free
layer, combined with the shift in said frequency due to the
magnetic field produced by the current in the bit line. Operation
is implemented in a conventional magnetic random access memory
having a plurality of magnetoresisitive cells formed by an
intersection of a grid of word and bit lines, wherein an individual
cell within the grid can be selected and switched from one magnetic
state to another by the magnetic fields produced by the currents in
the word and bit lines.
Inventors: |
Gider; Savas (San Jose, CA),
Nikitin; Vladimir (Campbell, CA) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
32680849 |
Appl.
No.: |
10/335,671 |
Filed: |
January 2, 2003 |
Current U.S.
Class: |
365/158; 365/171;
365/189.09; 365/199 |
Current CPC
Class: |
G11C
11/16 (20130101) |
Current International
Class: |
G11C
11/15 (20060101); G11C 11/00 (20060101); G11C
11/02 (20060101); G11C 5/00 (20060101); H01L
43/08 (20060101); G11C 011/00 () |
Field of
Search: |
;365/158,171,189.01,189.09,199 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Van Thu
Attorney, Agent or Firm: Feece; Ron
Claims
What is claimed is:
1. A magnetic random access memory device, comprising: a
magnetoresistive cell having a first resonance frequency; a first
electrical line electrically connected with the magnetoresistive
cell; a second electrical line electrically connected with the
magnetoresistive cell; a first power source coupled with the first
line to provide an electrical current through the first line to
shift the resonance frequency of the magnetoresistive cell to a
second resonance frequency; an AC power source coupled with the
second electrical line, the AC power source generating a current
having a frequency substantially equal to the shifted resonance
frequency.
2. The magnetic random access memory as recited in claim 1 wherein
the first power source is voltage source having a DC bias.
3. The magnetic random access memory as recited in claim 1 wherein
the first power source is a current source having a DC bias.
4. The magnetic random access memory as recited in claim 1 wherein
the magnetoresistive cell is a magnetic tunnel junction cell.
5. The magnetic random access memory as recited in claim 1 wherein
the magnetoresistive cell is a giant magnetoresistive (GMR)
cell.
6. The magnetic random access memory as recited in claim 1 wherein
said first power source applies a DC current in a first direction,
and switches to apply current in a second direction.
7. A magnetic random access memory device, comprising: a
magnetoresistive cell having a first resonance frequency; means for
shifting the resonance frequency to a second resonance frequency;
means for supplying an alternating magnetic field adjacent to the
magnetoresistive element, the alternating magnetic field having a
frequency substantially equal to the second resonance
frequency.
8. The magnetic random access memory as recited in claim 7, further
comprising an electrical line disposed adjacent the
magnetoresistive cell and wherein the means for shifting the
resonance frequency of the magnetoresistive cell is a DC power
source, coupled with the electrical line for generating a current
therein.
9. The magnetic random access memory as recited in claim.7, further
comprising a first electrical line disposed adjacent the
magnetoresistive cell and a second electrical line disposed
adjacent the magnetoresistive cell and substantially perpendicular
to the first electrical line, and wherein the means for shifting
the resonance frequency of the magnetoresistive cell is a first
power source coupled with the first electrical line, and wherein
the means for generating an alternating field is an AC second power
source coupled with the second electrical line.
10. The magnetic random access memory as recited in claim 7,
wherein the magnetoresistive cell is a magnetic tunnel junction
cell.
11. The magnetic random access memory as recited in claim 7,
wherein the magnetoresistive cell is a giant magnetoresistive (GMR)
cell.
12. The magnetic random access memory as recited in claim 9 wherein
the first power source is a current source with a DC bias.
13. The magnetic random access memory as recited in claim 9 wherein
the first power source is a voltage source with a DC bias.
14. A method for switching a memory state in a magnetic random
access memory having first and second electrically conductive lines
electrically coupled to one another by a magnetoresistive cell
having a first magnetic resonance frequency, the method comprising
the step of: generating a current in the first line, the current
generating a magnetic field to shift the magnetic resonance
frequency of the magnetoresistive cell to a second resonance
frequency; and generating an alternating current in the second
electrically conductive line, the alternating current having a
frequency substantially equal to the second resonance
frequency.
15. The method as recited in claim 14, further comprising applying
the current in a first direction along the first electrical line
and switching the direction of the current to a second
direction.
16. The method as recited in claim 14, wherein the current
generated in the first electrical line has a DC component.
17. The method as recited in claim 14, wherein the magnetoresistive
cell has a magnetic magnetically free layer with a net magnetic
moment, and wherein the current generated in the first line has a
DC component that generates a magnetic field in a direction to
oppose the magnetic moment of the free layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic random access memory, and
more particularly to the efficient selection and switching of a
magnetic element within a magnetic random access memory array.
2. Background Art
The desired characteristics for computer memory are high speed, low
power consumption, non-volatility, high data density and low cost.
Dynamic Random Access Memory (DRAM) cells are fast and expend
little power, but have to be refreshed many times each second and
require complex structures which can make them relatively
expensive. Flash type EEPROM cells are nonvolatile, have low
sensing power, and can be constructed as a single device, but take
microseconds to write and milliseconds to erase, which makes them
too slow for many applications, especially for use in computer
memory. Conventional semiconductor memory cells such as DRAM, ROM,
and EEPROM have current flow in the plane of the cell, i.e.,
"horizontal", and therefore occupy a total surface area that is the
sum of the essential memory cell area plus the area for the
electrical contact regions, and therefore do not achieve the
theoretical minimum cell area
Magnet Random Access Memory is a promising candidate for computer
memory that can meet the above stated objectives while overcoming
many of the limitations of the above described devices. The
benefits of Magnetic Random Access Memory (MRAM) are discussed in
"The Science and Technology of Magnetoresistive Tunneling Memory",
IEEE Transactions on Nanotechnology, Vol. 1, No. 1, March 2002, by
B. N. Engel et al. A Magnetic Random Access Memory is essentially a
grid of electrically conductive bit lines and word lines. The bit
lines are parallel to one another and are perpendicular to the word
lines, which are also parallel with one another. A magnetoresistive
cell, disposed at the intersection of each word and bit line,
electrically connects a particular word line to a particular bit
line. By applying a voltage across the magnetoresistive cell from
the particular word line to the particular bit line, the magnetic
resistance, and memory state, of the magnetoresistive cell can be
determined. Although various types of magnetoresistive cells can
potentially be used in a MRAM array, such as for example a Current
Perpendicular to Plane Giant Magnetoresistance Element (CPP GMR),
most development efforts have focused on the use of Tunnel Valves
also known as Magnetic Tunnel Junction (MTJ) cells, due to their
potential for high resistance change, .delta.R/R. Therefore, while
the state of the MRAM art will be described with reference to MTJ
cells, it should be appreciated that many of the same principles
and challenges apply to MRAM arrays incorporating other types of
magnetoresistive cells as well.
A MTJ cell, in its most general sense includes first and second
ferromagnetic layers separated by a thin insulating layer known as
a tunnel barrier layer. One of the ferromagnetic layers has its
magnetization pinned in a predetermined direction while the other
is free to rotate under the influence of magnetic field. Quantum
mechanical tunneling of electrons through the tunnel barrier layer
is allowed or inhibited based on the relative alignments of the
magnetization of the free and pinned magnetic layers. If the
magnetization of the free layer is aligned parallel with that of
the pinned layer, then electrons will tend to pass through the
tunnel barrier layer; if the magnetizations of the two layers are
anti-parallel, then electrons will not as easily pass. Furthermore,
the MTJ is designed so that the magnetically free layer will have a
magnetic anisotropy, which will cause the magnetization of the free
layer to be most stable when in either of these two states, (ie.
parallel or anti-parallel to the pinned layer). In this way, once
the MTJ cell is placed in a particular magnetic state it will tend
to remain there until acted on by a magnetic field, thus providing
its non-volatility. Switching of the MTJ cell is generally
accomplished by causing a current to flow through both of the word
and bit lines.
The free layer of the cell at the intersection of the energized bit
and word lines will undergo magnetization reversal, provided that
the magnetic fields produced by the word and bit lines are
sufficiently high. The Stoner-Wohlfarth coherent rotation model
quite accurately describes the switching behavior of a typical free
layer in the MRAM cell. According to the model, the switching will
occur if the magnetic fields in the hard and easy axis directions
lie outside of the so-called astroid curve, as shown in FIG. 4A.
The operating currents are chosen such that they produce a magnetic
field at the selected cell that satisfies the switching conditions
for the cell, but not for any other cell along word or bit lines
(i.e. the fields for non-selected bits have to lie within the
astroid curve). However, the cells in the array do not have
identical magnetic properties. Variations in the bit size, shape,
aspect ratio, and crystalline anisotropy lead to a distribution of
the switching fields. This leads to a problem regarding bit
selectivity, i.e., for a given write current, some of the selected
bits will switch, while others might not. Simply increasing the
write current would allow switching of these hard-to-switch bits;
however, some of the non-selected bits will switch with such large
currents. This is illustrated in FIG. 4B. The inner and outer
astroids (A.sub.min and A.sub.max, respectively) show the extremes
in the switching distributions of the free layer in the MRAM array.
If the operating point O is chosen within A.sub.max, then only
cells within astroid A can be switched, while others outside
astroid A cannot be addressed. To address these cells, the
operating point has to be moved to point P outside of A.sub.max.
But in such a case, all the cells with distribution described by
A.sub.min that lie along the selected word or bit lines, will be
unintentionally switched.
Another factor that adversely affects bit selectivity, is the
magnetic field produced by the neighboring cells. These fields can
change the fields required to switch the selected bit at the chosen
operating point or lead to unintentional switching of the
non-selected bits. The problem of the bit selectivity is widely
recognized in the field, and numerous solutions have been proposed,
such as the use of thermally assisted writing as described in U.S.
Pat. No. 6,385,082 by D. W. Abraham et al., the use of offset bit
currents as described in U.S. Pat. No. 6,424,561 by Shaoping Le et
al., and the use of magnetic bias as described in U.S. Pat. No.
6,163,477 by L. T. Tran. However, the proposed solutions generally
compromise some other desirable aspects of MRAM, such as power
consumption or areal density.
Another problem encountered in MRAM is the magnetic stability of
the bits. As the size of the bits become smaller, they approach the
superparamagnetic limit. To increase the stability, one solution
requires increasing the magnetic anisotropy of the bits, but this
also requires higher magnetic fields to switch the bits and in
turn, requires larger power consumption.
SUMMARY OF THE INVENTION
The present invention allows a particular magnetoresistive cell
within a MRAM array to be selected and switched with a minimum of
power and also allows a particular cell to be selected for
switching while leaving other adjacent cells unaffected. The
invention takes advantage of the ferromagnetic resonance frequency
of the cell. By applying a DC current to one of the lines connected
to the cell, the resonance frequency of the cell is shifted. An AC
signal is then applied to the other of the lines connected to the
particular cell. This AC signal is generated at a frequency that is
generally the same as the shifted resonance frequency of the
selected cell and creates a magnetic field at that same frequency.
This magnetic field causes the magnetization of the free layer to
oscillate at its shifted resonance frequency, rendering it easily
switched with a minimum of energy.
The present invention provides significant advantages over the
prior art for at least a couple of reasons. First, according to one
aspect of the invention, a selected magnetoresistive cell can be
switched from one magnetic/resistance state to another with a
minimum of input energy. As discussed with reference to the prior
art, the MRAM array includes a grid made up of a set of parallel
word lines and a set of parallel bit lines, the word lines and bit
lines being generally perpendicular to one another. Each specific
word line is connected with a specific bit line by a
magnetoresistive cell. The magnetic state of the cell can be
controlled by causing a current to flow through the word and bit
lines associated with the selected cell. The currents generate
magnetic fields, which act upon the magnetic moment of the free
layer to rotate it from one direction to another. By applying an AC
current to a word line at the ferromagnetic resonance frequency,
the magnetizations of the cells along the word line are rotated by
a larger angle than with a DC current of the same amplitude.
Additionally, by applying a DC current to a bit line, the
ferromagnetic resonance frequency is shifted for all cells along
the bit line, thus allowing for greater selectivity of the cell at
the intersection with the word line.
Second, according to another aspect of the invention the resonance
frequency of the particular selected cell can be shifted so that,
by applying the above described AC signal at this shifted
frequency, the selected cell will oscillate at the resonance
frequency resulting in large magnetization rotation while other
adjacent cells whose resonance frequency has not been shifted will
oscillate with a much smaller amplitude. Shifting of the resonance
frequency of the selected cell can be accomplished by generating a
current having a DC bias through a bit line. By way of example, a
predetermined DC current can be applied to the bit line. This
generates a magnetic field along the easy axis of the free layer
which shifts the resonance frequency of the cells along the bit
line as approximately described by the formula ##EQU1##
where .gamma. is the gyromagnetic constant, M is the saturation
magnetization of the free layer, H.sub.Keff is the effective
anisotropy of the free layer, which can include both the
crystalline anisotropy and the shape anisotropy, and Hext is the
external field from the bit line. The above equation assumes that
both the magnetization and the external field lie in the plane of
the free layer and along the easy axis. The external field may be
either parallel or antiparallel to the magnetization.
Then an AC current having a frequency that is essentially the same
as the shifted resonance frequency of the selected cell can be
applied to the word line associated with that cell, generating a
magnetic field of the same frequency. It will be appreciated that,
even though all of the cells on that word line will be exposed to
the oscillating magnetic field, and all of the cells on the bit
line will experience a constant magnetic field Hext, the cell at
the intersection of the word and bit lines will have by far the
largest magnetization rotation since it will be the only cell being
driven at resonance.
For a fuller understanding of the nature and advantages of the
present invention, reference should be made to the following
detailed description taken together with the accompanying figures,
wherein like reference numerals refer to like elements.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a perspective view, not to scale, of a portion of a MRAM
array according to an embodiment of the invention;
FIG. 2 is a view taken along circle 2 of FIG. 1 shown enlarged and
not to scale;
FIG. 3 is an electrical schematic view of a MRAM array according to
an embodiment of the invention;
FIG. 4A is an ideal Stoner-Wohlfarth astroid curve for a single
domain element;
FIG. 4B is a set of astroid curves showing the distribution of
switching fields in an MRAM array and possible operating
points;
FIG. 5 is a typical ferromagnetic resonance response;
FIG. 6 is the zero field response and the shifted response in a
field;
FIG. 7 is a representation of the magnetic states of the cells
along the word and hit lines including the selected cell at the
intersection; and
FIG. 8 is the timing diagram of the word and bit currents along
with the magnetic response of the selected cell.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates an exemplary Magnetic Random Access Memory
(MRAM) array 100 according to a preferred embodiment of the
invention. A plurality of magnetic tunnel junction (MTJ) cells 102
are provided at intersections of a rectangular grid of electrically
conducive lines 104, 106. These electrically conductive lines
include a set of conductive lines that function as parallel word
lines 104, configured in a horizontal plane, and another set of
parallel bit lines 106, arranged generally perpendicular to the
word lines in another horizontal plane so that the word lines 104
and bit lines 106 form a grid and appear to intersect if viewed
from above. Although two word lines 104 and two bit lines 106 are
shown, one skilled in the art will recognize that the number of
such lines would typically be much larger. A MTJ cell 102 is formed
at each intersection of a word line 104 and a bit line 106 to
vertically interconnect the word line with the bit line. The MTJ
cell 102 can be switched between two possible resistance values,
which define its binary memory state. During a sensing or reading
operation of the array, a voltage is applied across the cell 102
between the word line 104 and bit line 106 corresponding to that
MTJ cell 102, and the resistance value (i.e., memory state) is
determined.
The vertical current path through the cell 102 permits the memory
cell to occupy a very small surface area. While not shown in FIG.
2, the array may be formed on a substrate, such as silicon, which
may contain other circuitry as well. In addition, an insulating
material (also not shown) usually separates the word lines 104 and
bit lines 106 in regions other than the intersecting regions.
With reference to FIG. 2, a MTJ cell 102 according to a preferred
embodiment of the invention includes a free layer 108. The magnetic
moment of the free layer 108 is indicated by arrow 117. The MTJ
cell 102 also includes a magnetically pinned ferromagnetic layer
116. Preferably, the pinned layer includes antiparallel (AP)
coupled first and second ferromagnetic layers 118, 120 separated by
an AP coupling layer 122. The first and second ferromagnetic pinned
layers 118, 120 will hereafter be referred to as AP1 and AP2
respectively. The AP1 layer 118 and AP2 layer 120 have
magnetizations that are pinned along an axis that is parallel with
the easy axis of the free layer 108 as indicated by arrows 123 and
125. The magnetization of AP2120 is strongly pinned through
exchange coupling with an antiferromagnetic (AFM) material layer
124 formed adjacent to the pinned layer 116, and the antiparallel
coupling keeps AP1 strongly pinned in the direction opposite AP2.
While several antiferromagnetic materials would be suitable, such
as for example FeMn or NiMn, the AFM layer 124 is preferably PtMn,
which possesses a desirable combination of corrosion resistance,
Curie temperature, and exchange coupling characteristics. While the
preferred embodiment has been described as having an AP coupled
pinned layer, those skilled in the art will recognize that a simple
single pinned layer could be used as well.
The word line 104 passes above the MTJ cell 102, adjacent to and in
electrical contact with the free layer 108 and in the same
direction as the easy axis of the free layer. The bit line 106
passes beneath the MTJ cell 102, adjacent to and in electrical
contact with the AFM layer, and runs along a direction that is
perpendicular to the direction of the word line 104 and to the easy
axis of magnetization of the free layer 108 and the pinned magnetic
moments of the pinned layer 116. A thin insulating tunnel barrier
layer 126 separates the free layer 108 from the pinned layer 116.
The tunnel barrier layer 126 is constructed of an insulating
material such as for example, alumina (Al.sub.2 O.sub.3).
When the magnetic moments of the second ferromagnetic free layer
108 and AP1118 are aligned in the same direction, the spins of
electrons passing through these layers are in the same direction,
which allows electrons to pass through the tunnel barrier 126 based
on what is known as the tunnel valve effect. When the magnetic
moment of the free layer 108 is opposite that of AP1, the electrons
of each layer tend to have opposite spins, which renders them
unable to pass easily through the tunnel layer 126. In other words,
when the magnetic moments of free layer 108 and AP1118 are in the
same direction, the tunnel barrier layer 126 acts as a conductor
and when the magnetic moments are opposite, the tunnel barrier 126
acts as an insulator. By applying a voltage across the tunnel
junction cell 102 between its associated word line 104 and bit line
106, its resistance can be determined, thereby allowing the memory
state of the tunnel junction cell 102 to be read.
With reference now to FIG. 3, which shows an electrical schematic
representation of the MRAM array 100, a DC current source 128
applies a DC current to a particular word line 106(b). The MTJ
cells 102, 103, represented schematically as variable resistors,
have inherent magnetic resonance frequencies associated with their
free layers 108 (FIG. 3) which are a function of materials and
geometry of the MTJ sensor. It should be appreciated that this will
apply as well if another sort of magnetoresistive sensor is used,
such as for example a CPP GMR. Since the MTJ sensors have
essentially the same shape and material makeup, they will also have
roughly the same inherent resonance frequency. The DC current
generated by the DC source 128 creates a magnetic field that shifts
the magnetic frequency of all of the MTJ cells associated with the
particular word line 106(a), according to the function ##EQU2##
With reference to FIG. 3, which shows an electrical schematic
representation of an MRAM array 100, the cells are read as in the
prior art. The cells are written as in the prior art, but instead
of DC currents being applied along both the word and bit lines, a
DC current is applied to the bit line and an AC current at the
ferromagnetic resonance frequency of the free layer is applied
along the word line. With reference to FIG. 5, the magnetization
rotation of the free layer will be maximum at the ferromagnetic
resonance frequency. This rotation can be much larger than possible
with a DC current of the same amplitude, thus requiring less power.
Additionally, applying a DC current beforehand to the bit line, the
resonance frequency of all the cells along the bit will be shifted
from their zero field frequencies. If the polarity of the bit
current is chosen such that the resultant magnetic field is
opposite the magnetization direction of the free layer, the
resonance frequency is shifted down from the zero field frequency,
as shown in FIG. 6. In principle, the frequency can be shifted much
farther down than it can be shifted up for the same field. With
reference to FIG. 7, if the frequency of the word line is chosen to
correspond to the shifted resonance frequency, the selected cell at
the intersection with the bit line will oscillate at the shifted
resonance frequency, the selected cell at the intersection with the
bit line will oscillate at the shifted resonance frequency,
resulting in large magnetization rotation. Meanwhile, other cells
on the word line will oscillate with much smaller amplitude because
they are driven off-resonance. The combination of a DC bit current
and an AC word current provides greater selectivity than in prior
art. FIG. 8 shows the details of exemplary timing of the bit and
word lines along with the components of magnetization of the free
layer. First, the DC current is applied to the bit line, and after
a small delay corresponding to the rise time of the bit line
current, the AC current is applied to the word one at the shifted
resonance frequency of the AC current. With each successive period,
the amplitude of the hard axis magnetization grows in amplitude and
eventually, the torque on the free layer due to the bit field will
switch the easy axis magnetization to the same direction as the bit
field.
In an alternative embodiment of the present invention, the current
in the bit line 106 could be initially generated in such a
direction that the magnetic field induced by the current would be
in the same direction as the initial magnetization of the free
layer. Then after the AC current has been applied to the word line
104 at the resonance frequency of the cell 103, the current in the
bit line 106 can be switched to flow in the opposite direction
causing the free layer magnetization to flip. This alternate could
provide improved timing of the switching, because the timing of the
switching of the memory state of the cell 103 could be precisely
controlled by the timing of the current switching in the bit line
106. However, this timing improvement could come at a cost of
decreased speed.
An additional enhancement of the aforementioned embodiments
includes a positive feedback circuit to tune the AC source to the
shifted resonance frequency. This enhancement corrects for small
variations in resonance frequency from cell to cell due to
variations in the effective anisotropy of each cell. Initially, the
selected cell is driven at a frequency close to the shifted
resonance frequency, and the amplitude of the free layer
oscillation is detected simultaneously by measuring the
magnetoresistance of the cell. The amplitude signal is coupled back
into a circuit providing positive feedback and gain, leading to
sustained oscillations at the peak of the resonance curve. Such
feedback circuits are well known in LC and quartz crystal
oscillators.
It should be appreciated that the present invention has been
described herein as the best embodiment contemplated and is by no
means meant to be exhaustive. While the present invention has been
particularly shown and described with reference to the preferred
embodiments, it will be understood by those skilled in the art that
various changes in form and detail may be made without departing
form the spirit and scope of the invention. For example, while the
invention has been described having MTJ cells with simple, single
layer free layers, an MRAM array using MTJ cells with antiparallel
coupled free and or simple single layer pinned layers could also
incorporate the present invention. Furthermore, the invention could
incorporate an antiparallel coupled free layer wherein the current
from the word line passes through the center of the AP coupled free
layer as described in commonly assigned patent application Ser. No.
10/263,495. In addition, the MRAM array could use giant
magnetoresisitive (GMR) cells, or potentially some other
magnetoresistive cell, rather than MTJ cells. Accordingly, the
disclosed embodiments are considered to be merely illustrative and
the invention should be limited in scope only by the appended
claims.
* * * * *